1. Field of the Invention
The present invention relates to matrix display devices and systems, and to driving or addressing methods for such display devices.
2. Description of the Related Art
Liquid crystal display devices are well known, and usually comprise a plurality of pixels arranged in an array of rows and columns.
Typically the pixels are addressed or driven as follows. The rows of pixels are selected one at a time. The pixels within the row currently selected are provided with respective display settings by virtue of respective data voltages being applied to each of the columns. Such data voltages are known by a number of names in the art, including data signals, video signals, image signals, drive voltages, column voltages, and so on.
Selection of each of the rows one by one, with driving of the columns as required during each row selection, provides display of one frame of the image being displayed. The display is then refreshed by a further frame being displayed in the same manner, and so on.
The level of a data voltage applied to a pixel determines how much light is output by that pixel by controlling the extent of the optical modulation effect of the liquid crystal layer in the pixel. It is known that due to capacitance effects and time-response of the liquid crystal layer, the liquid crystal layer can fail to reach the optical modulation condition it would reach in a steady-state situation for a given drive voltage by the end of the time the drive voltage is applied in the addressing scheme. A correction method called overdrive correction (ODC) (which may also be termed overdrive compensation) has been employed to alleviate this effect.
Under ODC, a pixel is driven at a higher or lower voltage level than the voltage level that would be required for steady-state operation, so that by the end of the relevant voltage application period, the voltage present across the pixel has reached a level estimated to be substantially equal to what the steady-state level should be. Further details of known ODC methods are described in U.S. Pat. No. 5,495,265 and WO 2004/013835, which are incorporated herein by reference.
The correction to be applied under ODC (i.e. how different the level of voltage applied to the pixel to achieve a given voltage across the liquid crystal layer of the pixel is from the given voltage) varies according to the liquid crystal panel design. Moreover the required correction varies according to what voltage level a pixel is at in the frame prior to that being corrected, and what voltage level is being sought in the present frame i.e. the current pixel data setting and the next pixel data setting (this is often referred to as a voltage pair). The correction required is typically calculated anew for each pixel for each frame. Thus, in conventional ODC schemes, it is required to have a frame buffer, so the voltage pairs can be determined, a look-up table comprising a matrix of many voltage pairs and many voltage settings (and possibly different panels) so the appropriate correction can be read-off for the determined voltage pair, and a processor for determining the correction from these items.
In addition, in order to implement ODC for grey level transitions toward or near the extremes of the liquid crystal transmission curve, additional buffers and/or increased selector matrix complexity in the panel driver IC are typically needed, resulting in increased silicon area and cost.
Liquid crystal displays often have a backlight, e.g. a fluorescent lamp, arranged such that such that light from the backlight passes through the pixels where it is modulated by the liquid crystal layer. US 2004/0012551 A1 describes a variable backlight control system employed in a driving scheme.
It is separately known to drive other liquid crystal panels with so-called black fields inserted between the picture image fields, i.e. a driving scheme is employed in which in each frame a pixel is driven for some of the time at a data voltage level and for the rest of the frame is driven in black mode, as described in U.S. Pat. No. 5,912,651 which is incorporated herein by reference. The visual effect perceived by a viewer is such that this approach can reduce the blurring effect of a moving image.
The present inventors have realised it would be desirable to provide ODC driving schemes for matrix display devices that alleviate or reduce the large amount of processing required with conventional ODC schemes. The present inventors have also realised it would be desirable to provide ODC driving schemes for matrix display devices that reduce the size of frame buffers and/or look-up tables as used in conventional ODC schemes.